Helping Signals Retain Their Individuality

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Signal integrity (SI) issues in- clude any effects that distort a signal. Early telephone systems and transatlantic cables had SI problems long before microprocessors and digital communications proliferated.

The term applies to all signals, both analog and digital, but has become closely associated with high-speed digital signal quality through familiar displays such as eye diagrams and bathtub curves. By considering amplitude and timing separately, the problem can be simplified, although assigning a specific cause to either effect can be difficult if you are working only with the corrupted signal.

In a presentation made to Lockheed engineers in 2000, Dr. Eric Bogatin listed more than 20 SI causes and effects, including terminations, emissions, attenuation, ground bounce, ringing, RC delay, IR drop, parasitics, capacitance, and stubs. From the group, he distilled four high-speed problems: impedance discontinuities that cause reflections and distortions, crosstalk, rail collapse, and EMI.

The increase in circuit speed that occurred during the 12 years since the Lockheed presentation has only made correct treatment of SI causes more important. At the time. Dr. Bogatin proposed six design techniques to maintain high-speed signal integrity: (1)

1. Slow down edges.

2. Minimize the length of all interconnects.

3. Use contollecl impedance lines and terminate.

4. Use continuous, closely spaced adjacent power and ground planes.

5. Use low dielectric-constant material

for signal layers.

6. Minimize loop mutual inductances between signal lines. (1)

If a signal's rise and fall times are significantly slower than the propagation delay through the circuit, reflections will be minimized. Also, slower edge speed reduces high-frequency crosstalk. Inserting series resistors is a simple edge-speed reduction approach that uses a circuit's stray capacitance to advantage.

When rise and fall times are comparable to or faster than the circuit propagation delay, controlled impedance lines become necessary. Parallel termination works well to eliminate reflections for point-to-point signals. In contrast, seriesterminated lines achieve the full signal amplitude at the receiving end only because of reflection. Interconnects can be particularly unfriendly to fast signal edges if the high frequencies see a significant impedance discontinuity.

High capacitance between a PCB's power and ground planes is desirable because it creates a distributed return path for high frequencies. …

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